There are several ways to convert radioactive decay energy into electricity. One of them is the simple accumulation of decay particle charge across the changing potential of a capacitor. The direct charge capacitor can be charged by either alpha or beta particles from nuclear decay sources. Although capacitors with alpha emitters can produce higher voltages, suppression of secondary electrons from the alpha sources requires an external electrical power supply. Anno, “A Direct-Energy Conversion Device Using Alpha Particles,” Nuclear News, 6, 3 (1962). This shortcoming of a conventional direct charge alpha capacitors has resulted in them requiring as much energy to operate as they can produce, which has limited their practical use despite the fact that they can provide conversion up to the megavolt range.
On the other hand, nuclear decay with negatron (β−) beta emission is of special interest and involves the conversion of a neutron to a proton, electron, and antineutrino, the last two of which are ejected from the nucleus. Therefore, there is conservation of charge and the kinetic energy of the electron can propel it to the collector plate while the newly created proton is left in the emitter plate. The direct charge beta capacitor is unique in comparison to other electric generators using radioactive decay energy in that the beta capacitor produces relatively high working voltage (kilovolts) and relatively low current and this energy conversion from electron kinetic energy to electric charge can be performed relatively efficiently. But because of the relatively high voltage and relatively low current, there has been little, if any, use of such capacitors in commercial electronic devices. Nevertheless, efforts to develop such capacitors, in particular pulsed capacitors with high energy density, continue because capacitor energy is a function of the square of its operating voltage.
Despite significant work on β− direct charge capacitors over the years, most if not all of the heretofore known capacitors remain relatively inefficient. Specifically, if efficiency of a direct charge capacitor is quantified as the ratio of useful electrical power to the thermal power of the isotopes used in capacitor, the best known experimental direct charge capacitors have had efficiencies of less than about two percent. See, Lazarenko et al., “Desk-size Nuclear Sources of the Electricity Energy,” Energoatomizdat, Russia (1992). Further, for solid dielectric direct charge beta capacitors, only the penetrating radiation of Sr-90/Y-90 has shown measurable results, and that at an efficiency of less than about one percent. Coleman, “Radioisotopic High Potential Low-Current Sources,” Nucleonics, December, (1953).
In view of the foregoing, a need still exists for a direct charge beta capacitor with improved efficiency.